An Adaptive Pulse-Width and Frequency Modulation Control Strategy for Light Load Efficiency Improvement in Half-Bridge LLC Converter with Wide Input Voltage Range

Half-bridge LLC converter is widely used in photovoltaic (PV) applications for its high efficiency, high frequency, and simple structure. Conventional half-bridge LLC converter using frequency modulation (FM) has a wide range of switching frequency variation under wide input voltage range conditions, resulting in large transformer volume and reduced efficiency at light loads. To solve the problem, an adaptive pulse-width and frequency modulation (APWFM) is proposed under light load conditions. When the output power is below the critical power, the switching frequency decreases linearly as the output power decreases, and the duty cycle is simultaneously modulated to achieve constant output voltage, so the switching frequency variation range is smaller compared to FM. The size and efficiency of the converter can be further optimized in a limited frequency range. Experimental results show that the load efficiency is improved in a 300W LLC prototype and the efficiency is improved by 5% at 10% load.